Medical imaging devices take a variety of forms, including nuclear medical imaging devices such as described in Applicant's co-pending patent applications, the entire contents of which are incorporated herein by reference. As known to those of skill in the art, when using nuclear medical imaging devices, such as SPECT devices, it is desirable to determine the amount of radiation attenuation attributable to the portions of the subject surrounding the area being imaged.
SPECT (Single Photon Emission Computed Tomography) allows imaging of a subject by detecting gamma emissions from the subject. In medical imaging, a compound including a radioactive isotope is administered to a patient, the compound reaches a subject of interest, such as an organ, and gamma emissions are detected from the subject.
Gamma imaging techniques, such as SPECT, are emission imaging techniques, in which the distribution of an emissive material (such as a radionuclide) are of interest. In contrast, x-ray imaging of a subject, such as x-ray CAT scanning, is a transmissive imaging method that allows the attenuation properties of the subject to be determined. Emission images of a subject are also modified by the attenuation properties of the subject. Hence, it would be extremely valuable to obtain transmission and emission images of the subject together, so that the emission image can be corrected for the attenuation factors.
SPECT imaging of the heart or other organs is often complicated by the surrounding tissue absorbing or otherwise attenuating the radioactive emissions in a non-uniform manner, hindering the accurate reconstruction of an image. An x-ray or gamma ray CAT scan (often referred to as a transmission scan) can be performed along with SPECT (emission) imaging, and the CAT scan data used to determine the attenuation attributable to the patient's body. However, as conventionally performed, this approach presents a number of problems. First, current approaches to measurement of tissue attenuation require performing a transmission CAT scan (with either x-rays or gamma rays) either before or after the SPECT emission scan. The subject must remain absolutely motionless between these two scans in order that the attenuation measurements correlate with the emission images. Despite a subject's best efforts, movement of internal structures (e.g. colon contents) will occur, often over just minutes.
In addition, CAT scan apparatus are designed to operate quickly and at a high photon flux, so as to provide a “snapshot.” SPECT imaging, on the other hand, is a low photon flux process that, by its nature, requires a much longer period in order to acquire enough data to form an image. As such, the patient's heart, lungs and other tissues move through a range of motion during the SPECT imaging session. This substantially reduces the correlation between the rapid CAT scan “snapshot” and the “time averaged” SPECT image.
Hence, it would be advantageous to collect the x-ray transmission signal and SPECT emission signals over a concurrent time span, so that subject motion has a similar effect on both the transmission and emission images. However, x-ray imaging is typically a faster process than SPECT imaging, and no previous apparatus has allowed concurrent x-ray and gamma imaging. Typically, an x-ray image is obtained quickly, then a gamma image is obtained relatively slowly, leading to serious problems in correlating the data.
In any case, interpretation of medical images can be problematic and it would be advantageous to have multiple images of a subject using different techniques, as long as the images can be readily and accurately interrelated. Hence, improved imaging techniques would be valuable, for example, to allow collection of improved imaging data, and more accurate diagnosis of patient problems.